EVIDENCE FOR SKELETAL DEVELOPMENTAL PATTERNS

Evidence for skeletal development in radiolaria has been obtained by (1) observing living organisms with light optics to record events during skeletal deposition, (2) reconstructing skeletal ontogenesis by analysis of stages of development inferred from plankton or sedimentary samples, and (3) examining fine structural evidence based on scanning and transmission electron microscopy of living organisms fixed during varying stages of skeletal deposition (e.g., Anderson and Bennett 1985, Anderson et al. 1987, 1988, 1989, Thurow and Anderson 1986).

Based on these types of evidence, Anderson and Swanberg (1981) proposed that most skeletons of polycystinea can be explained as a product of two forms of skeletal development: (1) bridge growth yielding latticed structures and frameworks with somewhat polygonal pores, and (2) rim growth forming perforated shells with nearly round pores. However, the final shape of the pore may not be fully indicative of the developmental process. Further evidence of early stages, or regions with incomplete development of the skeleton, are often needed to confirm the type of ontogenetic process (e.g., Anderson et al. 1988). Using this paradigm of bridge and rim growth, several species of fossil and living radiolaria have been examined to determine the contribution of bridge and rim growth to the mature skeletal form (e.g., Anderson 1983, Thurow and Anderson 1986, Amon et al. 1990). Among the collosphaerid radiolaria, both types of skeletal growth are observed within different species (Anderson 1983).

In addition, Anderson and Swanberg (1981) used fixed collosphaerid colonial radiolaria samples, to discover that the central capsules of these species divide by binary fission during proliferation within the colony. However, all of these stages of binary fission occurred early in development before silica deposition. The living cytoplasmic sheath (cytokalymma) that eventually deposits the silica was present during binary fission and exhibited a typical hour-glass shape surrounding the dividing central capsules (e.g., Figure 1).

Skeletal deposition within the cytokalymma occurred later after all the central capsules had divided and filled the gelatinous envelope enclosing the colony. There was no evidence that further binary division occurred after the porous siliceous skeleton had been deposited enclosing the central capsule. This raised the interesting question of whether proliferation of the central capsules ceased after the skeleton was deposited. If so, this would be a serious limitation to further growth of the colony and could limit repair if there is disruption or trauma to the colony. If there were no further division, then the number of skeletal-bearing central capsules would be fixed until the colony dissipated during swarmer production (presumably sexual reproductive phase).

However, evidence is presented here, based on fossil specimens and laboratory observation of living species, that collosphaerid central capsules may divide after the porous skeleton has been deposited around the central capsule. These data raise the interesting questions of whether this process occurs widely in colonial and solitary species and the significance this process holds for the interpretation of radiolarian abundance and life cycle strategies.